1. Field of the Invention
This invention relates to zoom optical systems and an image pickup apparatus using the same and, more particularly, to an optical system which comprises a plurality of optical elements of two types, one of which has a plurality of reflecting surfaces and the other of which has refracting surfaces alone, wherein, of the plurality of optical elements, at least two optical elements move in differential relation to effect zooming (to vary magnification). Still more particularly, this invention relates to zoom optical systems suited to be used in video cameras, still video cameras or copying machines.
2. Description of Related Art
The zoom optical systems for the image pickup apparatus have been known as constructed with refracting elements or lenses alone. These lenses are of the spherical or aspheric form of revolution symmetry and arranged on a common optical axis so that their surfaces take revolution symmetry with respect to the optical axis.
In the field of art of photographic objectives, there have been many previous proposals for utilizing reflecting surfaces such as convex or concave mirrors. It has been also known to provide an optical system which makes use of a reflecting system and a refracting system in conjunction. This optical system is well known as the catadioptric system.
FIG. 23 is a schematic diagram of an optical system composed of one concave mirror and one convex mirror, or so-called mirror optical system.
In the mirror optical system shown in FIG. 23, an axial light beam 104 coming from an object is reflected by the concave mirror 101. While being converged, the light beam 104 goes toward the object side. After having been reflected by the convex mirror 102, the light beam 104 forms an image on an image plane 103.
This mirror optical system is based on the configuration of the Cassegrain type of reflecting telescope. The aim of adopting it is to shorten the total length of the entire optical system compared with the long physical length of the refracting telescope, as the optical path is folded by using two reflecting mirrors as arranged in opposed relation.
Even for the objective lens system constituting part of the telescope, for the same reason, the Cassegrain type and many other types have come to be known which differ in the number and the construction and arrangement of reflecting mirrors in order to ever more shorten the total length of the entire system.
Up to now, effort has been made to shorten the total length of the photographic lens as it is usually unduly long. For this purpose, instead of some of its lens elements, mirrors are used to efficiently fold up the optical path. A compact optical system of mirror type is thus obtained.
In the Cassegrain type reflecting telescopes or like mirror optical systems, however, the use of the convex mirror 102 leads, in general case, to a problem that the object light beam 104 is shaded in part. This is attributable to the fact that the back of the convex mirror 102 lies within the domain of passage of the object light beam 104.
To solve this problem, the mirror may be decentered, thus permitting the domain of passage of the object light beam 104 to be cleared of the obstruction of the other parts of the optical system. In other words, the principal ray 106 of the object light beam 104 is set off from an optical axis 105. Such a mirror optical system, too, has previously been proposed.
FIG. 24 is a schematic diagram of a mirror optical system disclosed in U.S. Pat. No. 3,674,334, which has solved the above-described problem of shading in such a way that the mirrors of revolution symmetry with respect to the optical axis are cut off in part.
The mirror optical system shown in FIG. 24 comprises, in the order in which the light beam encounters, a concave mirror 111, a convex mirror 113 and a concave mirror 112. In the prototype design, these are of the forms shown by the double-dots and single-dash lines, or of revolution symmetry with respect to the optical axis 114. In actual practice, the concave mirror 111 is used in only the upper half on the paper of the optical axis 114, the convex mirror 113 in only the lower half and the concave mirror 112 in only a lower marginal portion, thereby bringing a principal ray 116 of the object light beam 115 away from the optical axis 114. The optical system is thus made free from the shading of the object light beam 115.
FIG. 25 is a schematic diagram of another mirror optical system disclosed in U.S. Pat. No. 5,063,586. In this mirror optical system, the mirrors are so arranged that their central axes set themselves off the optical axis of the system. By this arrangement, a principal ray of the object light beam is dislocated from the optical axis, thus solving the above-described problem.
In FIG. 25, an object to be photographed lies in a plane 121. Assuming that a line perpendicular to the plane 121 is an optical axis 127, it is found that, as the light beam encounters a convex mirror 122, a concave mirror 123, a convex mirror 124 and a concave mirror 125 successively in this order, the centers of area of their reflecting surfaces and their central axes (the lines connecting those centers with the respective centers of curvature of these reflecting surfaces) 122a, 123a, 124a and 125a are decentered from the optical axis 127. In FIG. 25, the amounts of decentering of such parameters and the radii of curvature of all the surfaces are appropriately determined to prevent the object light beam 128 from being shaded by the back of any one of the mirrors. An object image is thus formed on a focal plane 126 with high efficiency.
In addition, U.S. Pat. Nos. 4,737,021 and 4,265,510 even disclose similar systems freed from the shading effect either by using partial mirrors of revolution symmetry with respect to the optical axis or by arranging the central axes themselves of the mirrors in decentered relation from the optical axis.
Meanwhile, the catadioptric optical system using both of reflecting mirrors and refracting lenses can be made to have the function of varying the image magnification. As an example of this optical system, mention may be made of deep sky telescopes disclosed in, for example, U.S. Pat. Nos. 4,477,156 and 4,571,036, in which the image magnification is made variable by using a parabolic mirror in the main mirror in conjunction with the Erfle eye-piece.
It is also known to provide another zooming technique which moves two mirrors constituting part of the above-described mirror optical system in differential relation. By this technique, the image magnification (or focal length) of the optical system for photography is made variable.
For example, U.S. Pat. No. 4,812,030 discloses application of such a zooming technique to the Cassegrain type reflecting telescope shown in FIG. 23, wherein the separation from the concave mirror 101 to the convex mirror 102 and the separation from the convex mirror 102 to the image plane 103 are made variable relative to each other. Thus, a mirror optical system for photography capable of zooming is obtained.
FIG. 26 shows another example of an application disclosed in the above U.S. Pat. No. 4,812,030. Referring to FIG. 26, a light beam 138 from an object encounters a first concave mirror 131 and is reflected from its surface, becoming a converging light beam. The converging light beam goes toward the object side, and encounters a first convex mirror 132. Here, the light beam is reflected toward the image side, becoming an almost parallel light beam. The almost parallel light beam goes to a second convex mirror 134 and is reflected therefrom, becoming a diverging light beam. The diverging light beam encounters a concave mirror 135. Here, the light beam is reflected and becomes a converging light beam, focusing an image on an image plane 137.
In this optical system, the separation between the first concave mirror 131 and the first convex mirror 132 is made to vary, while the separation between the second convex mirror 134 and the second concave mirror 135 is made to vary simultaneously, so as to effect zooming. The focal length of the entirety of the mirror optical system is thus made variable.
Also, in U.S. Pat. No. 4,993,818, an image formed by the Cassegrain reflecting telescope shown in FIG. 23 is then re-focused by another mirror optical system provided in the rear stage, thereby forming a secondary image. The magnifying power of the mirror optical system for forming the secondary image is made variable. By this arrangement, the photographic system as a whole is provided with the capability of varying the image magnification.
These reflecting optical systems for photography have a great number of constituent parts. To obtain the required optical performance, it is necessary to increase the accuracy with which to set up the individual optical parts. In particular, because the positioning tolerance for the mirrors is severe, it is indispensable to adjust the position and angle of each mirror.
To solve this problem, a method has been proposed, for example, to construct the mirror system in the form of one block, thus avoiding an error from occurring when the optical parts are set up.
Heretofore, what are known as such a block having a large number of reflecting surfaces therein are, for example, optical prisms such as pentagonal roof prisms or Porro-prisms used in the viewfinder systems.
For these prisms, a plurality of reflecting surfaces are unified by the molding techniques. All these reflecting surfaces are, therefore, made up under the control of their relative positions with high accuracy, thus obviating the necessity of doing later adjustment of the relative positions of the assembled reflecting surface to one another. However, the main function of these prisms is to change the direction in which light advances for the purpose of inverting the image. Every reflecting surface has, therefore, to take the plane form.
On the other hand, there is also known an optical system in which curvature is imparted to the reflecting surface of the prism.
FIG. 27 is a schematic diagram showing the main parts of an observing optical system disclosed in U.S. Pat. No. 4,775,217. This observing optical system is used for observing the external field or landscape and, at the same time, presenting an information display of data and icons in overlapping relation on the landscape.
The rays of light 145 radiating from an information display body 141 are reflected from a surface 142, going to the object side until they encounter a half-mirror 143 of concave form. After having been reflected from the half-mirror 143, the light rays 145 become nearly parallel by the refractive power of the concave surface 143, and pass through the surface 142, reaching the eye 144 of the observer. So, the observer views an enlarged virtual image of the displayed data or icons.
Meanwhile, a light beam 146 from an object enters at a surface 147 which is nearly parallel with the reflecting surface 142, and is refracted there, arriving at the concave half-mirror surface 143. Since this surface 143 is coated with a half-permeable layer by the vacuum evaporation technique, part of the light beam 146 passes through the concave surface 143 and is refracted in transmitting the surface 142, entering the pupil 144 of the observer. So, the observer views the display image in overlapping relation on the external field or landscape.
FIG. 28 is a schematic diagram showing the main parts of another observing optical system disclosed in Japanese Laid-Open Patent Application No. Hei 2-297516. This observing optical system, too, is used for viewing the external field or landscape and, at the same time, looking the information on the display device as overlapping the view.
In this observing optical system, a light beam 154 from an information display body 150 enters a prism Pa at a flat surface 157 and is made incident on a parabolic reflecting surface 151. Being reflected from this surface 151, the light beam 154 converges and forms an image on a focal plane 156. During this time, the light beam 154 for display undergoes total reflection from the successive two parallel planes constituting part of the prism Pa, reaching the focal plane 156. By this arrangement, thinning of the optical system as a whole is achieved.
The display light beam 154 that has exited as a diverging beam from the focal plane 156 then proceeds while undergoing total reflection between the flat surfaces 157 and 158, until it encounters a half-mirror surface 152 of parabolic form. The light beam 154 is reflected from the half-mirror surface 152 and, at the same time, forms an enlarged virtual image of the display by its refractive power, becoming a nearly parallel beam. After having passed through the surface 157, the light beam 154 enters the pupil 153 of the observer. Thus, the observer looks at the display image on the background of the external field or landscape.
Meanwhile, an object light beam 155 from the external field passes through a flat surface 158b constituting a prism Pb, then passes through the parabolic half-mirror surface 152 and exits from the surface 157, reaching the eye 153 of the observer. So, the observer views the external field or landscape with the display image overlapping thereon.
Further, an optical element can be used on the reflecting surface of a prism. This is exemplified as an optical head for photo-pickup disclosed in, for example, Japanese Laid-Open Patent Applications Nos. Hei 5-12704 and Hei 6-139612. Such a head receives the light from a semiconductor laser, then reflects it from the Fresnel surface or hologram surface to form an image on a disk, and then conducts the reflected light from the disk to a detector.
The conventional optical system of the type which has refractive optical elements alone puts the stop inside thereof. In many cases, the entrance pupil lies deep in the optical system. The longer the separation between the stop and the entrance surface at the frontmost position, the larger the ray effective diameter of that entrance surface becomes. Further, there is a problem that, as the angle of view increases, the ray effective diameter of that entrance surface increases even more greatly.
The optical systems of the mirror type disclosed in the above U.S. Pat. Nos. 3,674,334, 5,063,586 and 4,265,510 have a common feature that all the reflecting mirrors are made decentered by respective different amounts of decentering. Hence, the mounting mechanism for the reflecting mirrors becomes very elaborate in structure. It is also very difficult to secure the setup tolerance.
The photographic optical systems having the zooming function disclosed in U.S. Pat. Nos. 4,812,030 and 4,993,818, too, have, in any case, a large number of constituent parts such as mirrors and lens elements for forming an image. To obtain satisfactory optical performance, therefore, it is necessary to set up all the optical parts in relation to one another with high accuracy.
Particularly for the reflecting mirrors, the tolerance for the relative position becomes severe. Therefore, it is also necessary to accurately adjust the position and orientation of each of the reflecting mirrors.
It should be also noted that the conventional reflecting type photographic optical systems are adapted for application to the so-called telephoto type of lens systems as this type has a long total length and a small field angle. To attain a photographic optical system which necessitates the field angles of from the standard lens to the wide-angle lens, because an increasing number of reflecting surfaces for correcting aberrations is required to be used, the parts must be manufactured to even higher precision accuracy and assembled with even severer a tolerance. Therefore, the production cost has to be sacrificed. Otherwise, the size of the entire system tends to increase greatly.
Also, the observing optical systems disclosed in the above U.S. Pat. No. 4,775,217 and Japanese Laid-Open Patent Application No. Hei 2-297516 each have an aim chiefly to produce the pupil image forming function such that, as the information display is positioned remotely of the observer""s eye, the light is conducted with high efficiency to the pupil of the observer. Another chief aim is to change the direction in which light advances. Concerning the positive use of the curvature-imparted reflecting surface in correcting aberrations, therefore, no technical ideas are directly disclosed.
Also, the optical systems for photo-pickup disclosed in the above Japanese Laid-Open Patent Applications Nos. Hei 5-12704 and Hei 6-139612 each limit its use to a detecting optical system. Therefore, these systems are unable to satisfy the imaging performance for photographic optical systems and particularly image pickup apparatus using a CCD or like area type image sensor.
It is an object of the invention to provide a zoom optical system and an image pickup apparatus using the same, wherein there are provided a plurality of optical elements which are constituted by an optical element in which a plurality of curved or flat reflecting surfaces are formed and an optical element composed only of coaxial refracting surfaces, and relative positions of at least two optical elements of the plurality of optical elements are varied to effect zooming, so that the zoom optical system as a whole is minimized in bulk and size, and, at the same time, the accuracy with which the reflecting surfaces are set up (or the assembling tolerance) that greatly affects the performance little differs from item to item.
Further, a stop is disposed either on the object side of the zoom optical system or adjacent to a light entrance surface at which a light beam first enters, and an object image is formed at least once within the zoom optical system. By this arrangement, despite the zoom optical system having a wide angular field, the effective diameter of every one of the optical elements is shortened. Moreover, a plurality of reflecting surfaces constituting the optical element are given appropriate refractive powers. At the same time, these reflecting surfaces are arranged in decentering relation to thereby fold the optical path in the zoom optical system to a desired shape without causing shading of a light beam within the zoom optical system. It is, therefore, another object of the invention to provide a zoom optical system of shortened total length in a certain direction and an image pickup apparatus using the same.
To attain the above objects, in accordance with one aspect of the invention, there is provided a zoom optical system, which comprises a plurality of optical elements including a first optical element having two refracting surfaces and a plurality of reflecting surfaces formed in a transparent body, being arranged such that a light beam enters an inside of the transparent body from one of the two refracting surfaces and, after being successively reflected from the plurality of reflecting surfaces, exits from the other of the two refracting surfaces, and/or a second optical element having a plurality of surface mirrors integrally formed and decentered relative to one another, being arranged such that an incident light beam exits therefrom after being successively reflected from reflecting surfaces of the plurality of surface mirrors, and a third optical element composed of a plurality of coaxial refracting surfaces, wherein an image of an object is formed through the plurality of optical elements, and zooming is effected by varying relative positions of at least two optical elements of the plurality of optical elements.
Of the other features, especial ones are as follows.
A stop is disposed on a light entrance side of the zoom optical system, or adjacent to a light entrance surface at which a light beam first enters.
Each of the at least two optical elements of which relative positions are varied has an entering reference axis and an exiting reference axis in parallel to each other.
The at least two optical elements of which relative positions are varied move on one movement plane in parallel to each other.
Each of the at least two optical elements of which relative positions are varied has an entering reference axis and an exiting reference axis oriented to the same direction.
One of the at least two optical elements of which relative positions are varied has an entering reference axis and an exiting reference axis oriented to the same direction, and another of the at least two optical elements of which relative positions are varied has an entering reference axis and an exiting reference axis oriented to opposite directions.
Each of the at least two optical elements of which relative positions are varied has an entering reference axis and an exiting reference axis oriented to opposite directions.
Focusing is effected by moving one of the at least two optical elements of which relative positions are varied.
Focusing is effected by moving an optical element other than the at least two optical elements of which relative positions are varied.
The zoom optical system forms at least once an object image at an intermediate point in an optical path thereof.
Of the plurality of reflecting surfaces, curved reflecting surfaces are all formed to anamorphic shapes.
All reference axes of the at least two optical elements of which relative positions are varied lie on one plane.
At least a part of reference axes of an optical element other than the at least two optical elements of which relative positions are varied lie on the one plane.
At least one optical element of the plurality of optical elements has a reflecting surface in which a normal line on the reflecting surface at an intersection point of a reference axis with the reflecting surface is inclined with respect to a movement plane on which the at least two optical elements of which relative positions are varied move.